Gear Choice

Gear Choice

There is often a debate as to which gear is the best for a particular corner.

A general rule of thumb is that the best gear at the time of throttle application is the one that provides maximum acceleration from apex to corner exit.  

Each transmission is different, but most transmissions have a 20% difference between 3rd and 4th gears.  Given the same speed, the engine in third gear will be running at a 20% higher RPM than in fourth.  

Example:  5800 RPM in the Corvette in 4th is about 7200 RPM in 3rd. 

This is a part of the actual HP and Torque curve for the Corvette GT3.  It has a very "flat" torque curve (blue line).  Most modern racing engines have a reasonably "flat" curve.  Lower performance engines have torque curves that fall off at higher RPM-racing engines less so.

The iRacing version of the LT6 engine has a max torque of 460 ft pounds at 6300 RPM. Let's estimate at 5800 RPM torque is 440 ft pounds. And, at 7200 RPM torque is 420 ft pounds.

Force=Mass times Acceleration.   Acceleration = Force divided by Mass.   

Transmissions essentially MULTIPLY torque so 3rd gear has 1.2 times the torque of 4th gear.

OK. The Corvette has 5% less torque at 7200 RPM than at 5800 RPM but 3rd gear increases the torque by 20%. Greatly simplified:  3rd gear at 7200 RPM has 14% more torque and acceleration than 4th gear at 5800 RPM.  So initially, the Corvette will accelerate significantly faster in 3rd gear.   (Torque actually increases from 5800 to 6300 RPM, so the average increase is probably closer to 10%.)

Now, redline is about 7800 RPM, so using 3rd gear at the apex with 7200 RPM will require upshifting much sooner than if 4th gear was used.  Shifting down to 3rd on corner entry and up to 4th before corner track out takes time.  The downshift tends to slow the car in corner entry and acceleration is reduced when upshifting.  These reductions in speed must be compared to the increase in acceleration by using the lower gear. 

Another factor to consider is that the 10-14% more torque in the lower gear, in addition to increasing acceleration, also increases the risk of wheel spin and traction control intervention. 

So, the only real answer as to what gear is best is only determined by lap time. Often, there is very little difference. 

The "keep it in 4th" driver will probably see a bit more understeer both in corner entry and exit. But, no time or mental concentration will be lost in shifting. The "downshift to 3rd" driver will see less understeer on corner entry and exit. If using 3rd results in an RPM below 6800 at the apex the time duration with higher torque at the wheels and higher acceleration will most probably more than offset any time lost in upshifting. 

The Corvette will not allow downshifting if it would overspeed the engine. In most cases, the advantage, if any of using 4th rather than 3rd is that often in the process of downshifting, speed and momentum is lost. This speed and momentum loss can be minimized by downshifting as close to the apex as possible--the increased load on the rear tires will rotate the car without having to scrub off speed with steering input at the apex. 

 Author's Note:  I generally use the lower gear only for a very brief period-just before the apex and just after for a "burst" of extra acceleration and assistance in getting the car to rotate without as much steering input. 

IMPORTANT: The discussion above takes on a whole different perspective when dealing with a low horsepower winged formula car like the F4.  In the F4, maintaining speed and momentum is of paramount importance. Corning grip increases substantially with higher speed-likewise corner grip decreases with lower speed. If you feel able to use a lower gear, you may be slowing down too much on corner entry. 

Keep in mind that aerodynamic forces from wings and underbody channels increase at the square of speed. For example a 5% higher speed increases downforce by 10%.